The present invention relates to a system for monitoring the pressure of tires of a vehicle, and for communication of pressure information to an operator within the vehicle. The system employs a tire pressure sensor, mounted inside a rim of each wheel of the vehicle, which alters tire pressure values in accordance with changes in temperature. Circuitry, mounted on the chassis in proximity to the wheel, receives temperature-compensated pressure information which is transmitted inductively from the wheel. Processing and display circuitry may perform one of two functions. The circuitry may enable the operator to consult the system selectively for active monitoring of tire pressure. Alternatively, the circuitry may require no operator action, in which case the system would output an alarm, such as a warning light or other indication, when pressure falls below an acceptable level.
According to the invention, circuitry is provided in associated with a piezoresistive pressure transducer to provide accurate. efficient, and reliable adjustment of detected pressure values to account for changes in temperature during vehicle operation. These and other features of the invention will be described below.
Various kinds of tire pressure sensor systems which provide automatic indication of tire pressure are Well known. Some devices, such as that disclosed in U.S Pat. No. 4,040,380, include small indicators, connected to a tire valve, Which pop up When tire pressure falls below a predetermined value. Other types of tire-mounted indications, as shown in U.S. Pat. Nos. 3,7I7,030 and 4,723,445, provide a visual display on the wheel itself.
The just-mentioned devices are mounted only on wheels of vehicles, and do not need associated circuitry mounted anywhere else on the vehicle to provide visual indication of tire pressure. However, in order for such devices to be useful, it is necessary to inspect the tires visually, so that the vehicle would have to be stopped, and a driver would have to get out of his or her car.
Other approaches to tire pressure sensing are known which provide pressure information to a driver within the vehicle. In order to provide such a remote indication, the circuitry or other structure mounted on the rim of the tire must somehow communicate with some other circuitry elsewhere within the vehicle, so that the information may be transmitted to the driver within the vehicle. Different approaches have been tried. One such approach employs a radio transmitter with a self-contained independent power supply within the rim, with a receiver mounted close to the tire on the chassis of the car. The received information is transmitted to the vehicle console, which contains some sort of indicator to tell the driver when tire pressure is unacceptable. Examples of this approach are found in U.S. Pat. Nos. 4,443,785, 4,384,482 and 4,048,614. Of course, a self-contained independent power supply, such as a battery can become drained, so that the remote device can become inoperative.
Other approaches, such as that shown in U.S. Pat. Nos. 3,938,077, 3,911,434, 3,723,966, 3,694,803 and 3,092,806 employ a very well known technique of tuned circuit alteration. In this approach, a primary coil is mounted on a vehicle chassis, and transmits energy at a frequency in the radio range, for example, 175 kHz. A secondary coil, mounted on the rim of the wheel, is excited by the transmission of the primary coil. In this manner, energy is transmitted to the secondary coil for operation of the sensing equipment in the wheel. Responsive to an indication of tire pressure a capacitor may be mounted across the ends of the secondary coil and periodically short circuit the coil. The short circuiting of the secondary coil affects the operation of the primary coil, and the effect in operation may be translated into an indication of tire pressure within the vehicle itself.
Remote tire pressure sensors which transmit and receive energy between the vehicle and the wheel may do so in a periodic manner, or in a continuous manner. The difference depends primarily on the structure of the rim-mounted antenna or transmitter circuitry. One approach disclosed in U.S. Pat. No. 3,786,413 operates only when the vehicle is in motion. Where the circuitry is located at a single place on the rim, the circuitry can make contact with chassis-mounted circuitry only once per revolution of the wheel. However, if an oscillator antenna were looped around the rim, a chassis-mounted primary coil could be in continuous contact with the secondary coil on the rim. Most of the just-mentioned U.S. patents, including U.S. Pat. Nos. 3,723,966 and 3,092,806, provide examples of this. Other examples are found in the following U.S. Pat. Nos. 4,609,905; 4,567,460; 4,562,874; 4,554,528; 4,529,961; 4,510,484; 4,450,431; 4,409,586; 4,389,884; 4,363,020; 4,348,654; 4,130,817; 4,064,482; 4,057,783; 4,020,456; 4,017,826; 4,006,449; 3,990,041; 3,950,726; 3,930,224; 3,922,639; 3,913,065; 3,858,174; and 3,806,905.
One problem that arises during operation of tire pressure sensors of any kind is that tires heat up as they are run for longer periods of time. When a tire heats up, air expands within the closed volume of the tire, thus causing increased pressure within the tire, though the overall amount of air within the tire remains the same. Since the pressure nominally is different, a tire pressure sensor can provide different pressure readings when a tire is hot than would be the case if the tire were cold. This is why tire and vehicle manufacturers recommend that owners check their tire pressure when the tire is cold. Of course, with a remote tire pressure sensor, an operator may receive a continuous indication of tire pressure within the vehicle, but the indication may be inaccurate because of the temperature change. Thus, it is necessary to compensate for changes in temperature.
There are known approaches to compensating for temperature changes within a tire. One example of a device providing a temperature-compensated pressure indication is shown in PCT Published Patent Application No. W087/00127, published Jan. 15, 1987, having as an equivalent U.S. Pat. No. 4,703,650. In this device, a piezoresistive transducer has a continuous voltage applied thereto, and provides a pressure indication which, while compensating to some extent for temperature changes nonetheless incorporates some temperature drift which is inherent in the transducer itself. To compensate for this drift, a thermistor is provided to correct the indications given by the transducer. The corrected signal indications are provided in digital form as a pulsed output. It is possible to derive indications of both temperature and pressure from the coded signal output.
The just-mentioned U.S. patent provides separate indications of temperature and pressure as a single coded signal. U.K. Published Patent Application No. 2112757 provides a similar indication, to enable readout of separate temperature and pressure indications for each tire. Great Britain Published Patent Application No. 1301359 also provides such separate indications.
It also is known to compensate for ambient temperature outside the tire, as evidenced for example by U.S. Pat. No. 4,567,459. This additional compensation can have an effect on the pressure reading obtained, since changes in temperature outside the tire, as well as those inside the tire, affect the amount of "flexing.infin. of a tire, and thus affect the volume within a tire.
There is another PCT Published Patent Application, No. W087/00129, having as an equivalent U.S. Pat. No. 4,737,761, which discloses a transmission means employing inductive coupling, for controlling transmission and reception of the coded signal output by the device in U.S. Pat. No. 4,703,650.
Other known devices also provide temperature compensation. Examples of such devices are disclosed in U.S. Pat. Nos. 4,310,220, 4,052,696, and 4,465,075 teaches an on-chip pressure transducer which also employs thermistors to compensate for temperature drift. The above-mentioned U.S. Pat. No. 4,723,445 also addresses the issue of the need for temperature compensation.
While the first-mentioned PCT application, as well as the other temperature-compensation approaches, disclose useful ways of providing temperature-compensated pressure indications, these systems contain some inherent inaccuracy in measurement, and inefficiency in transmission, which the present invention corrects.